Carbon-coated electron-transmission window



1965 R. E. HESTER ETAL 3,211,937

CARBON-COATED ELECTRON-TRANSMISSION WINDOW Filed April 20, 1962 INVENTORS Ross E H55 TER WILLIAM A. SHE/ WOOD A TTOR/VEY United States Patent O 3,211,937 CARBON-COATED ELECTRON-TRANSMISSION WINDOW Ross E. Hester, San Lorenzo, and William A. Sherwood, Livermore, Califi, assignors to the United States of America as represented by the United States Atomic Energy Commission Filed Apr. 20, 1962, Ser. No. 189,216 4 Claims. (Cl. 313--74) The present invention relates to electron beam windows and, more particularly, to a carbon coated window for the transmission of extremely high-energy electron beams between regions of different pressures. More specifically stated, the present invention provides a foil window whereby an electron beam of the order of hundreds of amperes at 5 mev. is transmitted from a relatively low-pressure region to a high-pressure region.

Many of the present sciences, such as radiation chemistry, physics, medicine, metallurgy, and others, are finding ever increasing uses for high-energy electrons. These energetic electrons may be provided by acceleration to the desired energy in an evacuated tube. The electrons are then permitted to issue from the tube through an appropriate window.

These electron windows, then, need be pervious to the electron beam and yet impervious to the molecules in the region to which the beam issues. The window must enable the electron beam to pass from the evacuated tube to a different pressured region, while at the same time maintaining the pressure difference. Accordingly, these windows are commonly formed from such low atomicnumbered materials as aluminum or beryllium.

The desire to perform certain operations in conjunction with these beam windows, however, results in the following dilemmas.

Firstly, the high-energy electron beam sets up a charge density on the surface of the beam window. The interaction of this surface charge and electric beam intensity produces a stress on the window which is commonly balanced by increasing the thickness of the foil window.

On the other hand, the atomic particles constituting the foil window present cross-sectional areas that scatter the electrons in the oncoming beam. The angle of this scattering is commonly reduced by decreasing the thickness of the foil window. Thus, these conflicting alternatives prevent experiments requiring low scattering of the electrons.

Another problem basic to the art of electron-beam windows involves thermal stresses. It is difiicult to solder these extremely thin films in place, yet the windows must be secured to the evacuated acceleration tube in such a manner that the junction is not only vacuum-tight, but also of good thermal conductivity, i.e., heat generated in the foil by the passage of the high-energy electrons therethrough must be conducted away to a heat sink.

Yet another problem, that of structural support, arises. The foil must be thick enough to withstand the force per unit of area due to the pressure difference between the separated regions, yet thin enough for the electrons to issue therethrough with minimum loss of energy.

The present invention overcomes the above-noted difficulties in a unique manner, i.e., coating carbon on a preferred embodiment of the foil, namely aluminum oxide. This thin, carbon coating provides the conductivity necessary to relieve the electrical stress on the foil window without overly increasing the thickness of the window and, consequently, the scattering angle. Further, this allows the transmitted beam to be of higher quality and intensity than was heretofore possible.

Now a process has been invented that provides the 3,211,937 Patented Oct. 12, 1965 aforesaid thin, aluminum oxide, carbon-coated window and a mounting surface for same.

It is a primary object of the present invention, therefore, to provide a thin electron-beam window that is operable for extremely high-energy electrons.

It is a further object to provide a carbon-coated foil Window of large diameter and only 2,000 A. in thickness that will transmit an extremely high-energy electron beam of the order of 200 amperes between different pressure regions.

Yet another object of the present. invention is to provide a process for the manufacture of a carbon-coated, electron-beam window such that said window is in good thermal contact with a heat sink, and thereby capable of dissipating the heat arising from the electron bombardment.

Other objects and advantages of the present invention will be more readily ascertained from an inspection of the following specification taken in connection with the accompanying drawing wherein like numerals refer to like parts throughout, while the features of novelty will be more distinctly pointed out in the appended claims.

In order that the process be better understood, reference is made to the drawing, of which FIGURES 1 through 6 illustrate successive steps of the process.

The present invention may be further understood by referring to FIGURE 7, a partial cross-sectional, exploded view of the present invention in its preferred mounting.

Briefly, the process comprises fine-finishing one side of an aluminum plate, machining indentations into said plate, anodizing said machined plate, removing this anodized layer from the indented portion of the aforementioned plate, removing the remaining aluminum from the indentation, and coating carbon onto the aluminum oxide layer adhered to the non-indented side of said plate.

In step I of the process, as shown in FIGURE 1, an aluminum plate 11 is given a hand-lapped, fine finish on one surface 12 thereof.

In step II of the process, as shown in FIGURE 2, a right-cylindrical cavity 13 is machined out of the non fine-finished side of aforementioned plate 11. In the center of cavity 13, a right-cylindrical cut (creating the hole 14) is made to a depth such that only a thin film of aluminum remains in the bottom of the cavity. For purposes that are further described, infra, a chamfer is cut off the top of the hole 14. During this operation, the exterior of the plate is also formed to disc shape for ultimate clamping to the wall separating the different pressured volumes. The exterior need not, of course, be made circular, but it usually is.

In the next step of the process, shown in FIGURE 3, the aforesaid plate, including the indentation, is anodized. One means of anodizing the plate is to place same in a 3% solution of ammonium citrate connected as an electrode to a 200 v., 2 a. battery. From basic laws of physics, it is known that the aluminum oxide thus formed on the plate is directly proportional to the voltage and the time for which the voltage is applied. These variables are controlled such that a thin layer of aluminum oxide 16 is formed on the plate surface.

In step IV of the process, as shown in FIGURE 4, the aluminum oxide is removed from the inner area of the right-cylindrical cavity 14. This removal is done by swabbing the base of said cavity with sodium hydroxide.

In step V of the process, as shown in FIGURE 5, the anodized disc is cleaned and then dipped in 8-normal hydrochloric acid for seven or eight minutes to remove the exposed layer of aluminum at the base of cavity 14. It is at this time that the previously mentioned chamfer becomes useful; for in drying the window, this allows the acids to drain uniformly therefrom.

The final step of the pl'OCeSS, as shown in FIGURE 6, lvolves vacuum-plating a carbon layer 17, to a thickness ven less than that of layer 16, on the non-indented side f the disc. This layer 17 is boiled on by means common the art. By initially fine-finishing the carboned side of 1c p1ate,'per step I, a much more uniform oxide layer formed. This, in turn, produces a carbon coating of a .niform thickness.

Referring to FIGURE 7, there is shown a preferred mbodi-ment of the present invention 18, sealed to a porion of the wall 19 of an evacuated chamber by means of clamping ring 21 which is clamped against wall 19 by plurality of bolts 22 which extend through ring 21 and .re then threaded into wall 19. The curve-d portion of he beam window 18 is explained by a curing of the oil due to electron-beam bombardment 23. During op- :ration of the accelerator, this bombardment causes the gradual stretching of the foil until it assumes a partial ipherical shape. This curved surface, as is well known, s better capable of withstanding pressure.

While the present invention has been described in deail with respect to one embodiment thereof, it will, of :ourse, be apparent that numerous modifications may be nade within the spirit and scope of the invention, and it s therefore not desired to limit the invention to the exact details shown, except insofar as they are defined in the following claims.

What is claimed is:

1. A beam window for transferring an electron particle beam between different pressured regions, said beam being substantially cylindrical, comprising (a) a thin foil, impervious to gas and pervious to said particle beam and positioned across said beam, and (b) a thin continuous coating of carbon adhere to said foil. 2. An electron-beam window in accordance with claim 1 wherein said foil comprises a metal foil of low atomic number.

3. An electron beam window in accordance with claim 1 wherein said foil comprises a foil of aluminum oxide.

4. An electron-beam window for transferring a highenergy electron particle beam of the order of 200 amperes at 5 mev. between different pressured regions comprising (a) a thin foil of aluminum oxide, A1 0 and (b) a thin continuous coating of carbon adhered to said foil, the total foil thickness of said beam window being less than 2,000 A.

References Cited by the Examiner UNITED STATES PATENTS 1,907,507 5/33 Coolidge 3 l374 2,722,620 11/55 Gale 3l374 3,054,175 9/62 Spreter 29527 3,089,235 5/63 Boulet et al 29--527 GEORGE N. WESTBY, Primary Examiner. 

1. A BEAM WINDOW FOR TRANSFERRING AN ELECTRON PARTICLE BEAM BETWEEN DIFFERENT PRESSURED REGIONS, SAID BEAM BEING SUBSTANTIALLY CYLINDRICAL, COMPRISING (A) A THIN FOIL, IMPERVIOUS TO GAS AND PERVIOUS TO SAID PARTICLE BEAM AND POSITIONED ACROSS SAID BEAM, AND (B) A THIN CONTINUOUS COATING OF CARBON ADHERE TO SAID FOIL. 